Environmental concerns have resulted in increasingly strict regulation of engine emissions by governmental agencies. For example, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important and current indications are that this trend will continue.
Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. In the past, the emission levels of US diesel engines have been regulated according to the EPA using the Federal Test Procedure (FTP) cycle, with a subset of more restrictive emission standards for California via the California Air Resources Board (CARB). Proposed Tier II emission standards, are 50% lower than current Tier I standards. Car and light truck emissions are measured over the FTP 75 test and expressed in gm/mi.
Regulatory agencies continue to propose and apply stricter emission standards. For example, proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi. NOx and 0.08 gm/mi. particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and Ultra-Low Emission Vehicles (ULEVs) in California have to meet a 0.05 gm/mi. NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi. also have to be met. The EPA has also proposed tighter regulations for off-road diesel engines, requiring them to emit 90% less particulate matter and nitrogen oxides, by 2014 than they do today.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping, by themselves will not be able to achieve these low emission levels required by the standard. Aftertreatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine.
Some promising aftertreatment technologies designed to meet future NOx emission standards include lean NOx catalysts, Selective Catalytic Reduction (SCR) catalysts, and Plasma Assisted Catalytic Reduction (PACR). Current lean NOx catalyst technologies can reduce engine out NOx emissions in the range of 10 to 30 percent under typical operating conditions. One limitation that these technologies share is that catalytic surfaces within each device require periodic regeneration in order for the devices to continue to function properly. And regeneration usually involves supply a source of reductants, for example, urea or diesel fuel.
The use of urea has limitations. Using urea to regenerate catalysts requires a system for the storage and supply of urea in ready proximity to the engine. This requirement is especially limiting when the engine is used in transportation. Under these circumstances a network of urea supplies must be available to replenish the vehicle's onboard urea stocks as they are depleted.
The use of diesel fuel to regenerate aftertreatment system catalyses is also problematic. For example these systems often involve a significant fuel penalty, as a portion of the engine's fuel supply must be diverted to the aftertreatment system when aftertreatment catalysis regeneration is required. If the fuel is delivered during periods of high exhaust output a large portion of fuel must be introduced into the system to create exhaust gas rich in hydrocarbon.
Another complication is that there are only a finite number or reaction sites on the surface of any catalyst. Once all of these reaction sites are occupied excess reactants will not associate with the catalyst and will not react. This is especially problematic when one or more of the reactants flow across the surface of the catalyst. For example, depending upon parameters such as exhaust pipe diameter, exhaust gas flow rate, catalyst surface area and the amount of reductant in the exhaust gas, a significant amount of the reductant in the rich exhaust gas may pass over the catalyst surface un-reacted. In addition to contributing to the fuel penalty associated with regenerating the catalyst, any amount of hydrocarbon fuel that is vented to the atmosphere is itself a pollutant.
Therefore, there is a need for an engine aftertreatment system that provides a ready source of reductants to regenerate exhaust aftertreatment system components and that does not result in a significant fuel penalty or the release of un-reacted reductant into the atmosphere. Some aspects of the present invention are directed toward addressing this need.